To generate white light by a light-emitting diode (LED), a semiconductor chip is conventionally used which emits a narrow-band blue light. Portions of the emitted blue light are converted into yellow and red light components by a converter, e.g., in the form of particles, e.g., fluorescent particles. White light may be generated from the color mixture of unconverted blue light and converted yellow and red light. For the light conversion and generation of the color mixture, the converter particles should be in the optical path, i.e., the optical path of the emitted light of the LED.
A conventional method of generating converter particles in the optical path of an LED is volume casting in which converter particles are distributed within a silicone matrix, the silicone matrix being, e.g., distributed around the LED.
A further conventional method to generate converter particles within the optical path of an LED is the use of fluorescent layers comprising fluorescent substances in a silicone matrix and being produced by a screen process. These fluorescent layers may then be fixed to an LED, e.g., by a glue.
A further conventional method of generating converter particles in the optical path of an LED is electrophoretic deposition of fluorescent layers on an LED.
A further conventional method of generating converter particles in the optical path of an LED consists in spraying fluorescent substances or, respectively, converter particles onto a substrate, e.g., a semiconductor chip, e.g., an LED. Thereby, a suspension or dispersion of a fluorescent substance in a low viscous matrix, e.g., a silicone material or an epoxy resin in an uncured state, is sprayed onto a substrate through a nozzle by over pressure. By repeatedly spraying the substrate with the fluorescent suspension or fluorescent dispersion, the layer thickness of the matrix containing the fluorescent substance and thus the chromaticity coordinate or, respectively, the color mixture of the emitted light may be adjusted. The spraying-on of the fluorescent suspension or fluorescent dispersion may be set up for one single LED, a panel of several, e.g., several hundred, LEDs or a chip wafer, wherein the LEDs on the panel or chip wafer may be singularized upon being sprayed with on the fluorescent substances.
Due to the spraying, the fluorescent layer on a panel or chip wafer may exhibit inhomogeneities, e.g., differing chromaticity coordinates, i.e., the emitted color (color mixture) may have different configurations for the LEDs on a panel or a chip wafer. The reason for the inhomogeneity of the chromaticity coordinate on a panel or chip wafer may be spreading or segregation of already sprayed-on fluorescent material, which may locally result in the forming of different thicknesses of the fluorescent layer on the substrate. A spreading of the sprayed-on layers when the matrix is in an uncured state, e.g., silicone or epoxy resin in an uncured state, may be configured by the viscosity of the substance of the matrix.
The matrix material should be as highly viscous as possible, but still sprayable, wherein the fluorescent suspension or dispersion becomes more difficult with increasing viscosity of the fluorescent suspension or fluorescent dispersion.
Furthermore, fluorescent particles already deposited on the substrate may be removed and/or shifted by the impulse of further sprayed-on fluorescent particles. Moreover, the result may be an agglomeration, i.e., an accumulation, clustering or clotting of fluorescent particles. Furthermore, it may be difficult to provide more complex substrate kinds such as an LED housing or package with a fluorescent layer by spraying. Due to the above-mentioned problems, process control may be exacerbated, thus reducing reproducibility and homogeneity of the color distribution in a finished product. Furthermore, highly viscous substances may not be used for the matrix in the targeted range of thickness of the fluorescent layer, i.e., in an area of approximately 100 nm to approximately 200 μm.